1. Field of the Invention
This invention relates to a multiuser communication system and in particular to a multiuser detection (MUD) receiver for jointly demodulating co-channel interfering digital signals using estimates of the parameters of each individual signal, as distorted by their unique propagation channels, such estimates being generated by a parameter estimator.
2. Description of Related Art
Prior art methods for multiuser detection are included in the textbook “Multiuser Detection, Cambridge University Press, 1998 by Verdu. Verdu describes several different types of multiuser detectors, but assumes that the parameters of the individual signals are known a priori, and that the signature waveforms (one of the required parameters) do not extend past the symbol boundaries.
Other prior art methods for multiuser detection are described in U.S. patent application Ser. No. 09/923,709, filed by Rachel Learned et al. on Aug. 7, 2001, entitled “Method for Overusing Frequencies to Permit Simultaneous Transmission of Signals From Two or More Users on the Same Frequency and Time Slot”, and in U.S. patent application Ser. No. 09/943,770, filed by Rachel Learned on Mar. 28, 2002, entitled “Systems for Parameter Estimation and Tracking of Interfering Digitally Modulated Signals”. These patent applications estimate the parameters by assuming that signals are added to the propagation channel one at a time, but give no method for estimating parameters when the channel is always occupied by multiple users. In addition, Learned makes the assumption that the shape of the signature waveform is known, although it is often unknown due to multipath and other dispersive channel propagation effects.
U.S. Pat. No. 5,790,606 issued Aug. 4, 1998 to Paul W. Dent and assigned to Ericsson Inc., of North Carolina, entitled “Joint Demodulation Using Spatial Maximum Likelihood” discloses a type of multiuser communication system that uses several antennas which receive overlapping co-channel transmissions from several users (i.e. cell phones). Unfortunately, Dent's design will not work when the bit transitions of the various co-channel transmitters are not aligned in time at every antenna (a virtually impossible condition to meet). In virtually all real world applications, the digital signal is passed through a filter, which smoothes the rectangular digital signal and extends its influence into neighboring symbols. This intersymbol interference (ISI) then must be accounted for when attempting to jointly demodulate a co-channel aggregate signal. In addition, ISI can also be caused by multipath and other dispersive channel propagation effects. These effects are normally mitigated through the use of adaptive equalizers, but these equalizers do not work in the co-channel interfering signal case. The second drawback of this approach is that it requires multiple (and usually a large number) of antennas.
U.S. Pat. No. 6,122,269, issued to Wales on Sep. 19, 2000 performs multiuser detection and parameter estimation for a packet radio application. This procedure uses MUD to jointly demodulate packets that have unintentionally collided in time. The procedure uses known symbol sequences to solve for the unknown channel impulse response coefficients, and a correlation process to locate the positions of the known symbol sequences. In the case of short “snapshots” (vectors of received waveform samples), the correlation process will produce noisy data, and inaccurate known symbol sequence position estimates. In addition, the waveforms correlated against do not include the (unknown) channel impulse response, and will therefore also be adversely affected by leaving those out of the correlation equation. In addition, there is no mention of a method to determine the number of users which are colliding at any given time, and which users are colliding (as identified by their unique known symbol sequences).